Vitamin E (tocopherol) is the major fat-soluble antioxidant in humans. However, surprisingly little is known about the fate of this molecule once it enters target cells, or about the molecular events by which its storage, metabolism, and biological functions are regulated. As tocopherols are poorly soluble in water, it is believed that specific binding proteins are critical mediators of vitamin E function by virtue of their ability to solubilize and transport their ligand in the aqueous environment of the cell. Here, we propose to study the tocopherol transfer protein (aTTP) in order to gain insights into its biological role(s) and, in turn, into the mode of action of vitamin E. We will use our combined expertise in synthetic chemistry, stable-isotope analysis, protein biochemistry, and intra-cellular signaling to elucidate the functions of aTTP in mediating the biological activities of vitamin E. Specifically, we propose to address three specific aims: 1)- to characterize the structure/function relationship in aTTP in vitro. Using novel optical activity assays and site-directed mutagenesis, we will obtain molecular- level understanding of ligand binding and transfer activities of aTTP, and of the naturally occurring mutations in this protein. 2)- To characterize the structure/function relationship in aTTP in vivo. We will examine the effects of naturally occurring mutations in aTTP on tocopherol secretion from cultured hepatocytes. 3)- To identify proteins that interact with aTTP in a tocopherol dependent manner, and to determine the sub-cellular localization of the protein. Studies proposed in this application will provide critical information that is likely to improve our understanding of vitamin E function as a mediator of human health.